1. Field of the Invention
The present invention is directed to a process for applying a tough, flexible polyurethane coating to a substrate containing aramid fibers and to the coated substrate obtained by this process.
2. Description of the Prior Art
The use of composites as lightweight alternatives to steel has been steadily increasing in recent years. The strength-to-weight and rigidity-to-weight ratios of these composites are many times higher than those of steel. Skins of epoxy impregnated aramid fibers such as Kevlar aramid fibers available from DuPont are used to reinforce the composites, especially those having a honeycomb core such as Nomex honeycomb core, available from DuPont.
Prior to the use of these composites, they are generally coated in order to protect the composite from the enviroment in which it is used. Since the composites are extensively used in aircraft, severe requirements are placed upon the coatings. For example, the composites used in aircraft are subjected to repeated temperature cycles from as low as -65.degree. F. to as high as 160.degree. F. These repeated temperature fluctuations result in stresses to the the skin as well as the coating due to the different thermal and moisture expansion properties for the components of the skin, especially the aramid fibers.
Because the fibers expand at a different rate than the surrounding matrix, stresses are created during the repeated temperature changes resulting in microcracks in the aramid fiber-containing skin. Microcracks are also developed in the coating used on the skin if it cannot withstand these internal stresses. Further, once microcracks develop in both the skin and the coating, water can enter the honeycomb core through these microcracks and degrade the properties of the composite.
Accordingly, it is an object of the present invention to develop a coating for a substrate containing aramid fibers which can be used with composites such that the composites will be able to withstand repeated and wide variations in temperature without developing microcracks. The coating must also possess all of the other properties previously required of coatings of this type such as adhesion to the surface of the composite and solvent resistance. Surprisingly, these objectives may be achieved in accordance with the present invention discussed hereinafter.